1. Field of the Invention
The present invention relates to a technique of processing substrates, such as semiconductor wafers and glass LCD substrates, in processing vessels by using a processing gas, such as ozone gas, more specifically, a technique of purging the interior of the processing vessel after the processing with the processing gas has been completed.
2. Description of the Related Art
Fabrication processes for semiconductor devices include the step of applying a photoresist to a semiconductor wafer, the step of transferring a circuit pattern to a photoresist by photolithography process, the step of developing the photoresist and the step of removing the photoresist from the wafer.
Generally, the resist removing step is performed by dipping wafers in a cleaning tank filled with a chemical liquid called SPM (mixed liquid of H2SO4/H2O2).
However, from the ecological viewpoint, recently the resist removal using ozone aqueous solution whose waste liquid treatment is easy has become prevalent. In this resist removal, wafers are dipped in a cleaning tank filled with ozone aqueous solution to oxidize the resist with radicals of oxygen atoms in the aqueous solution so as to decompose the resist into carbon dioxide, water, etc. In the conventional method for such resist removal, a high concentration of ozone gas is bubbled in pure water to produce ozone aqueous solution, the ozone aqueous solution is fed into the cleaning tank through a piping. However, this method has a disadvantage that while ozone aqueous solution is being fed to the cleaning tank and also during a time from the load of the ozone aqueous solution to the start of the processing, the ozone concentration in the aqueous solution is decreased. Ozone (radicals) of the ozone aqueous solution present in the vicinity of the wafers react with the resist to be extinguished, but the supplementation of ozone is not quickly performed. As a result, a sufficient amount of ozone cannot be fed to the resist surface, and accordingly the reaction speed is not high.
Then, as an innovational substitute of the above-described dip cleaning, a cleaning method using ozone gas and steam has been proposed. This cleaning method includes the following steps (1) to (5) which are sequentially performed: (1) a step of feeding hot air into a processing vessel to raise the wafer temperature; (2) a pre-pressuring step of feeding ozone gas (or ozone gas and steam) to pre-pressurize the interior of the processing vessel; (3) an O3/steam processing step of feeding ozone gas and steam into the processing vessel to process the wafers; (4) an O2 purging step of purging the interior of an ozone gas feed pipe with oxygen gas; and (5) an air purging step of feeding cool air into the processing vessel to purge the interior of the processing vessel with the cool air.
In the O2 purging step, oxygen gas, which is a raw material gas of ozone gas, is introduced into an ozone gas generator with the ozone gas generator being stopped. The interior of the ozone gas generator and the interior of the pipe between the ozone gas generator and the processing vessel are purged with the oxygen gas. Accordingly, the O2 purging step takes a considerably long time. Furthermore, in the air purging step, ozone stagnates and resides at areas located outside of the main stream of the air formed by jetting the cool air. This disadvantageously reduces the effect of the purging step. Consequently, the throughput of the apparatus is low.